This invention relates to analog to digital conversion circuits used in imaging, and more specifically to programmable gain and offset for different colors in the Analog Front End (AFE) of a color digital camera, scanner or other imager.
This invention in particular is directed to the analog to digital converter subsystem part of an imaging system which includes a light source, a light focusing element, an image detector, an analog to digital converter, a controller chip, and memory. This invention interacts with the controller chip and memory in the imaging system to reduce the errors of the overall system caused by the light source, light focusing element and image detector.
It is common for the lighting and the focusing system within an imaging system to have non-ideal, non-linear characteristics. U.S. Pat. Nos. 6,174,649, 5,499,112, 5,808,295, 6,299,329, 6,357,904 show different methods for improving the light source linearity. The U.S. Pat. No. 5,499,112 best summarizes these non-idealities in its review of prior art in FIGS.  1–16.
In scanning applications, the linear tube light, which scans the document, has non-uniform light intensity near its two ends as opposed to its mid section. Also, the lensing system, which focuses the image onto a CCD linear array, has distortions near the two ends of a linear lighting system. Finally, at initial power ON of scanner lighting system and also over time through aging, the light intensity over the image has a non-uniform and time dependent nature.
In a typical imaging system, an Analog Front End (AFE) circuit is used with a CCD, CMOS or other image sensor. The AFE will provide initial amplification and calibration of the signal before it is digitized. In particular, a Correlated Double Sampler (CDS) samples the analog signal, and also samples a reset value and a black level (with no light). A Programmable Gain Amplifier (PGA) amplifies each pixel value before it is provided to an Analog-to-Digital Converter (ADC). Different color values are typically provided by using different color filters in front of the pixel of the image sensor.
Different color values require amplification by different amounts because the image sensors have different responses for different colors. In addition, sometimes multiple green pixels are used because the human eye is more sensitive to green. The use of multiple pixel values allows enhancement.
The particular color value presented at a pixel in a line can be programmed as well. A color filter array pattern is defined by programming pixel repeat registers and line pattern registers. The line pattern can be a different pattern of repeating colors, such as the Bayer pattern or the CYMG (cyan, magenta, yellow, green) pattern. In addition to putting the pattern in the line pattern register, the pixel repeat register is used to indicate how many pixels are used for each pattern before the pattern repeats.
Once the color filter array pattern has been defined, the intensity detected for each pixel can be associated with a particular color, and the gain can be programmed accordingly. In an example circuit, the National Semiconductor LM98501, multiple registers are provided to allow the programming of different gain values for different colors. A typical situation where this occurs is where the camera detects different light levels. At different levels of light, the required amplification of the image sensor signal can vary. A combination of all the colors is used to produce white. At different levels of brightness of the ambient light, the amplifications of each color must be varied so that they will combine to produce white. This is typically done automatically in a digital camera, which has a processor which detects the light level and changes the amplification registers for the different colors accordingly.
A typical approach to offset and gain is to use a single value for all the pixels in a line or image, or a single value for each of multiple colors for the entire line or image. For example, Exar CCD image digitizer XRD9861 uses such a fixed gain over the whole line. Another method involves changing the offset and gain in a predictable repeating pattern. This is intended to adjust for color offsets, not fringe effects. An example is shown in Exar's CCD image digitizer XRD9863. Another method used in some prior art devices is to use an analog input, rather than a digital input for the offset in gain.
In another pending application of the same assignee, a master gain register is used, with different values being used for each of the other colors. Thus, when the ambient light level changes, only the master gain register needs to be changed. This is used for adjusting video in digital cameras, for example. The application is entitled “Programmable Pixel Gain Control with Master Gain”, Ser. No. 09/680,800 filed Oct. 4, 2000.